Method and apparatus for protecting radio frequency power amplifiers

ABSTRACT

A power amplifier protection system to sense one or more operating conditions of a power amplifier and regulate the power of the input signal to the power amplifier as a response to one or more of the sensed operating conditions. This dynamic protection system may protect a power amplifier from failure by monitoring such operating conditions as power output and temperature and reducing the amplitude of the input signal if the power amplifier is operating beyond specified levels.

FIELD

[0001] The present invention pertains to power amplifiers and inparticular to a method and apparatus to protect power amplifiers fromfailure caused by operational conditions.

BACKGROUND OF THE INVENTION

[0002] Radio frequency (RF) power amplifiers are known in the art foruse in amplifying RF signals for broadcasting purposes, including radioand television. These amplifiers may be employed for broadcasting eitheranalog or digital signals. In some applications, such as mobilecommunication systems, RF power amplifiers are frequently cycled, turnedOn and Off, during operation. The average power of these amplifiers isdefined as the product of the duty cycle and the peak power. As the dutycycle of a RF power amplifier increases, so does its average power. Asthe average power of an amplifier increases it may be exposed tooperating conditions which causes or accelerates its failure.

[0003] Typically, power amplifiers are designed to operate withincertain current, voltage, temperature, and frequency limits. Operating apower amplifier beyond its designed limits exposes the amplifier tofailure. Thus, power amplifier manufacturers typically specify theoperating parameters or conditions for each particular type of poweramplifier, including power dissipation and operating temperatures.

[0004] Manufacturers also typically over-design power amplifiers towithstand wide power and temperature variations. Designing such ruggedpower amplifiers tends to make them larger than otherwise necessary,more expensive, and may degrade their RF performance. Additionally, evenruggedly designed power amplifiers are susceptible to certain types offailures.

[0005] One such type of failure may occur when the output of the poweramplifier is mismatched. That is, power amplifiers are often designed tooperate with a particular impedance or termination value. For example,the output of a power amplifier may be designed to be coupled to anominal 50 Ohm load or termination, such as an antenna with impedance of50 Ohms. However, in an actual implementation, the power amplifier maybe terminated with a substantially different terminating impedancevalue. Such mismatched termination may result in the power amplifierdissipating more power internally than what it was designed todissipate. This may result in additional heat being generated which maycause the power amplifier to operate at temperatures higher than what itwas designed to operate.

[0006] Another circumstance which may cause a power amplifier to failmay occur when the source and/or input voltage to the power amplifierchanges during operation. That is, a power amplifier may be designed tooperate within a certain voltage range; for instance 2.8 to 3.4 VoltsDC. However, during operation, the source and/or input voltage to thepower amplifier may increase beyond such range; for instance 5.0 VoltsDC. Such increase in voltage may cause the power amplifier to dissipatemore power internally, resulting in more heat being generated andincreasing its operating temperature. This may result in the poweramplifier operating beyond its designed limits and lead to its failure.

[0007] Accordingly, there is a need for a method and/or apparatus toprotect power amplifiers from failure caused by mismatched loads and/orlarge supply voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1A shows a system-level diagram illustrating a firstembodiment of the power amplifier protection system of the presentinvention.

[0009]FIG. 1B shows a system-level diagram illustrating a secondembodiment of the power amplifier protection system of the presentinvention.

[0010]FIG. 1C shows a system-level diagram illustrating a thirdembodiment of the power amplifier protection system of the presentinvention.

[0011]FIG. 1D shows a system-level diagram illustrating a fourthembodiment of the power amplifier protection system of the presentinvention.

[0012]FIG. 1E shows a system-level diagram illustrating a fifthembodiment of the power amplifier protection system of the presentinvention.

[0013]FIG. 2 shows a flow chart illustrating one method of practicingthe power amplifier protection system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Referring to FIG. 1A, a high-level system diagram is shownillustrating a first embodiment the power amplifier protection system ofthe present invention. A power amplifier 102 amplifies a given inputsignal 104 and provides an amplified output signal 106. In oneembodiment, the power amplifier 102 may be a radio frequency (RF)non-linear power amplifier which may be employed in transmitting anynumber of broadcast signals at various transmission frequencies. Inanother embodiment, the RF power amplifier may be a linear poweramplifier. According to various embodiments, the power amplifier may bedesigned to operate with a predetermined load impedance value, todissipate no more than a maximum measure of power, and/or to operatebelow a maximum temperature level.

[0015] The present invention provides a protection system to guardagainst power amplifier failure due to excessive power dissipationand/or operating temperatures. A feedback loop is employed for thispurpose, wherein the power of the input signal to the power amplifier,or one stage of the power amplifier, is regulated, varied, or reduced tocause the power amplifier to operate within specified limits whennecessary. This failure protection system may be comprised of aplurality of components.

[0016] One or more sensing means or devices may be employed to sensecertain operating conditions of a power amplifier. For instance, thepresent invention may provide a current sensing means 110 and a voltagesensing means 112 to sense or measure the current and voltage at theoutput of the power amplifier 102. The current and voltage sensing means110 and 112 may be any one of a number of circuits and/or devicesconventionally known to those skilled in these arts.

[0017] In various embodiments, the current sensing means 110 may sensethe current drawn from the amplifier's power source, the current througha transistor in the power amplifier, and/or the output current of thepower amplifier. According to one embodiment, the current sensing means110 may comprise a resistor of known value such that the current isproportional to the voltage across the resistor. In a second embodiment,a conventional current mirror comprising two transistors in parallel maybe employed. With said current mirror, the collector currents may beproportional to the emitter areas. The two transistors may be selectedsuch that one transistor has a small emitter area in comparison to thesecond transistor. Thus, the transistor with the smallest emitter areawould function as a current sensing means without draining anysignificant power from the power amplifier 110.

[0018] In one embodiment, the current sensing means may provide avoltage corresponding to the current sensed. In a second embodiment, thecurrent sensing means may provide a voltage that is linearlyproportional to the current sensed.

[0019] According to one implementation, the voltage sensing means 112may comprise a rectifier, or other peak-detection device, to provide thepeak output voltage. In a second implementation, the voltage sensingmeans 112 may comprise a circuit or device to measure the average outputvoltage.

[0020] The present invention may further provide a temperature sensingmeans 114 to sense or measure the operating temperature of the poweramplifier 110. The internal temperature and/or external temperature ofthe power amplifier 110 may be sensed, measured, and/or quantified bythe temperature sensing means 114. The temperature sensing means 114 maybe any one of a number of conventional temperature sensing or measuringcircuits or devices. According to one embodiment, the temperaturesensing means 114 may comprise one or more transistors configured tosense a voltage which is proportional to absolute temperature.

[0021] The voltages, currents, and temperatures sensed, measured, orquantified, may then be utilized to determine if the power amplifier isoperating beyond acceptable, desired, or specified limits or levels. Ifany one of the operating conditions is beyond the desired operatinglevel or threshold value, then a feedback system may regulate,dynamically or otherwise, the power of the input signal to the poweramplifier.

[0022] The current sensing means 110, which provides a voltagecorresponding to the current sensed, is coupled to a first inputterminal of a first differential amplifier 122 while a first referencevoltage source 116 is coupled to a second input terminal of the firstdifferential amplifier 122. The first reference voltage source 116 mayprovide a threshold voltage corresponding to a desired current throughthe power amplifier 102. In one implementation, this threshold voltagemay correspond to the maximum output current specified for the poweramplifier 102.

[0023] Similarly, the voltage sensing means 112 is coupled to a firstinput terminal of a second differential amplifier 124 while a secondreference voltage source 118 is coupled to a second input terminal ofthe second differential amplifier 124. The second reference voltagesource 118 may provide a threshold voltage corresponding to a desiredoutput voltage for the power amplifier 102. In one implementation, thisthreshold voltage may correspond to the maximum output voltage specifiedfor the power amplifier 102.

[0024] Likewise, the temperature sensing means 114, which may provide avoltage corresponding to the sensed temperature, is coupled to a firstinput of a third differential amplifier 126 while a third referencevoltage source 120 is coupled to a second input of the thirddifferential amplifier 126. The third reference voltage source 120 mayprovide a threshold voltage corresponding to a desired operatingtemperature for the power amplifier 102. In one implementation, thisthreshold voltage may correspond to the maximum operating temperaturespecified for the power amplifier 102.

[0025] Any of the reference voltage sources 116, 118, and 120 may beconfigurable and/or variable. In various implementations, one or more ofthe reference voltage sources 116, 118, and 120 may provide apredetermined threshold voltage level or a dynamically changing voltagelevel.

[0026] While the sensed conditions do not exceed their correspondingreference or threshold voltage levels, the differential amplifier doesnot provide a regulating feedback signal to the power amplifier.

[0027] According to one embodiment, each differential amplifier 122,124, and 126 may provide a first output voltage level if the voltage atits first input terminal is less than the voltage at its second inputterminal. Otherwise, each differential amplifier may provide a secondoutput voltage level if the voltage at its first input terminal isgreater than or equal to the voltage at its second input terminal. Themagnitude of the second output voltage level may vary during operationaccording to a change in the voltage at the first input terminal of thedifferential amplifier 122, 124, and 126.

[0028] In one embodiment, the first, second, and third differentialamplifiers 122, 124, and 126 may provide a fixed first output voltagelevel, zero volts for example, if the voltages corresponding to thesensed current 110, voltage 112, and temperature 114 are less than thecorresponding voltage at the first, second, and third reference voltagesources 116, 118, and 120. Otherwise, if one or more of the voltagescorresponding to the sensed current 110, voltage 112, and/or temperature114 is greater than or equal to the corresponding reference voltages atthe first, second, and third reference voltage sources 116, 118, and120, then the corresponding differential amplifier may provide apositive, or negative, voltage output.

[0029] In one embodiment, the output terminals of the differentialamplifiers 122, 124, and 126 are coupled to a maximum follower means128. The maximum follower means 128 may be any one of a number ofconventionally known circuits or devices which is capable of selecting,from among a plurality of inputs, a single input with the greatestabsolute voltage value, and providing said greatest voltage value at anoutput terminal of the maximum follower means 128. According to oneembodiment, the maximum follower means 128 may comprise three emitterfollowers whose emitters are coupled together.

[0030] The output of the maximum follower means 128 may be coupled to aloop amplifier means 130. The loop amplifier means 130 may be any one ofa number of conventionally known voltage amplifiers.

[0031] According to one implementation, the loop amplifier means 130may, in turn, be coupled to a variable gain amplifier (VGA) 132 so as tovary the power of a signal passing to and/or through the VGA 132. TheVGA 132 may receive an input signal 104 and provide the signal, or amodified version of the signal, as its output. The VGA 132 may be anyone of a number of conventionally known variable gain amplifiers capableof varying the power of a signal passing through the VGA 132. In oneimplementation, the VGA 132 may vary the signal power by reducing theamplitude of the signal. In a second embodiment, the VGA 132 may varythe signal power by offsetting its DC voltage. In a third embodiment,the VGA 132 may vary both the signal amplitude and DC voltage offset toreduce the input signal power.

[0032] According to one embodiment, the output of the VGA 132 may becoupled to the input terminal of the power amplifier 102 to provide aninput signal to the power amplifier 102. In this manner, the sensingmeans 110, 112, and 114, differential amplifier means 122, 124, and 126,maximum follower means 128, loop amplifier 130, and VGA 132 providefeedback compensation to the power amplifier 102. Such feedback loop mayfunction to vary the power and/or amplitude of the input signal to thepower amplifier 102 so that it operates within specified powerdissipation and/or temperature levels.

[0033]FIG. 1B shows a second embodiment of the present inventionillustrating how the present invention may be practiced in a multi-stagepower amplifier. In this configuration, a first stage amplifier 134 hasits output coupled to the input of a variable gain amplifier (VGA) 132which, in turn, has its output coupled to the input of a second stageamplifier 102. The current sensing means 110 and voltage sensing means112, coupled to the second stage amplifier 102, and a temperaturesensing means 114 provide feedback to the VGA 132. By said feedback, theamplitude or DC offset level of the signal passing through the VGA 132may be varied so that the power amplifier operates 102 within specifiedpower dissipation and/or temperature levels.

[0034] The feedback circuit, including differential amplifiers 222, 224,and 126, reference voltage sources 116, 118, and 120, maximum follower228, and loop amplifier 130, operate as described with reference to FIG.1A.

[0035] While the embodiment shown in FIG. 1B shows a VGA 132 between twostages of an amplifier, the present invention is not limited by thenumber of stages of an amplifier nor by the placement of the VGA 132within said amplifier.

[0036] For instance, FIG. 1C illustrates another embodiment of thepresent invention where an amplifier with at least three-stages isemployed. The output terminal of a first stage amplifier 134 may becoupled to the input terminal of a VGA 132. The output terminal of theVGA 132 may be coupled to the input terminal of a second stage amplifier136. The output terminal of the second stage amplifier 136 may becoupled to the input terminal of a third stage amplifier 102. Thisinvention may be practiced with a power amplifier comprising a pluralityof stages, even in excess of the number of stages illustrated.

[0037] The present invention is not limited by the number of sensingmeans and may be practiced with one or more sensing means. According toone embodiment, shown in FIG. 1C, two sensing means, a voltage sensingmeans 112 and a current sensing means 110, may be employed. In variousother embodiments, the invention may include a voltage sensing means, acurrent sensing means, and/or a temperature sensing means or anycombination of the three. Other sensing means, such as a flux sensingmeans, may also be employed to gather the desired operating conditionsof a power amplifier. In one embodiment, the output power of the poweramplifier is sensed by a forward directional coupler. In the case wherea single sensing means is employed, the maximum follower means 140 maybe removed from the feedback circuit.

[0038] Additionally, the present invention may provide a power controlmeans 138 as part of the failure protection system. The power controlmeans 138 may be accomplished using any one of the sensing meansemployed by the power amplifier. According to one configuration, thismay be accomplished by coupling the current sensing means 110 to a firstinput terminal of a differential amplifier 122 and coupling a referencevoltage source 138 to a second input terminal of the differentialamplifier 122. The reference voltage source 138 may be configurableand/or variable so that it may be preset and/or dynamically adjusted.While the voltage level the first input terminal is less than thevoltage level at the second input terminal, the differential amplifier122 may provide a first voltage level at its output terminal. Otherwise,if the voltage level at the first input terminal is greater than orequal to the voltage level at the second input terminal, then thedifferential amplifier 122 may provide a second voltage level at itsoutput terminal. In one implementation, the first voltage level may bezero volts while the second voltage level may be the same voltage levelas is present at the first input terminal of the differential amplifier122.

[0039] The output terminal of the differential amplifier 122 may becoupled to the VGA 132, either directly or indirectly, for instance viaa maximum follower means 128 and loop amplifier means 140, to the VGA132 to vary the power of the signal. In one embodiment, the signal powermay be varied by changing the amplitude of the signal so that the poweramplifier operates within certain power dissipation and/or temperaturelevels.

[0040] The present invention is also not limited to the placement of theVGA 132 between certain stages of a power amplifier. In variousembodiments of the present invention which utilize a VGA, the VGA may beplaced anywhere along the signal path either before, after, or betweenone or more power amplifier stages.

[0041] In other embodiments, the present invention may be practicedwithout employing a VGA along the feedback path. As shown in FIG. 1D,the protection system may operate directly on a power amplifier stage160 to vary the gain of the amplifier stage 160 and affect the power ofthe signal passing through the amplifier stage 160. In one embodiment,the feedback signal to the power amplifier stage 160, may operate tooffset the signal's DC bias point at that stage. According to a secondembodiment, the power amplifier stage 160 may incorporate the functionsof a variable gain amplifier. Note that the power amplifier stage 160may be any stage along the power amplifier.

[0042] Although a few embodiments of the power amplifier protectionsystem of the present invention have been illustrated, this system maybe implemented in numerous other forms without deviating from theinvention. For instance, as shown in FIG. 1E, the loop compensationamplifier may be built into the differential amplifiers 152, 154, and156 thereby removing the need for a separate loop compensation amplifierafter the maximum follower means 158.

[0043] In various embodiments, the power amplifier protection system ofthe present invention may be a separate device or module or beincorporated within a power amplifier device.

[0044]FIG. 2 illustrates one method of practicing the power amplifierprotection system of the present invention. According to thisembodiment, the desired operating conditions are sensed or measured. Forinstance, the power amplifier's output voltage 202, current passingthrough the amplifier 204, and/or operating temperature 206 values maybe obtained. One or more of these values may be compared to somereference operating values to determine if the power amplifier isoperating outside the desired limits 208. If it is not, then the failureprotection system repeats this process. If any one of the operatingconditions is outside the desired limits, then the power of the inputsignal to the power amplifier is modified or varied to bring the poweramplifier within the desired operating limits 210. In one embodiment,varying or regulating the power of the input signal may comprisereducing the amplitude of the input signal.

[0045] The power amplifier protection system described herein may becompletely or partially practiced in various forms including, but notlimited to, integrated circuit devices, programmable devices,machine-readable media, and/or a software program.

[0046] While the invention has been described and illustrated in detail,it is to be clearly understood that this is intended by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of this invention being limited only bythe terms of the following claims.

What is claimed is:
 1. An apparatus comprising: a power amplifierincluding an input terminal and an output terminal, the input terminalto receive an input signal; and a protection circuit coupled to thepower amplifier to sense one or more operating conditions of the poweramplifier and configured to regulate the power of the input signal tothe power amplifier as a response to one or more of the sensed operatingconditions.
 2. The apparatus of claim 1 wherein the power amplifier is aradio frequency non-linear power amplifier.
 3. The apparatus of claim 1wherein the protection circuit regulates the power of the input signalif one of the sensed operating conditions is beyond a desired operatinglevel.
 4. The apparatus of claim 3 wherein the protection circuit doesnot regulate the power of the input signal otherwise.
 5. The apparatusof claim 3 wherein the protection circuit regulates the power of theinput signal via a feedback signal.
 6. The apparatus of claim 3 whereinthe desired operating level corresponding to the one or more operatingconditions is the maximum operating level at which the power amplifieris specified to operate.
 7. The apparatus of claim 3 wherein if the oneor more sensed operating conditions indicate that the power amplifier isoperating beyond a specified power level, then the protection circuitcauses the power of the input signal to the power amplifier to bereduced.
 8. The apparatus of claim 3 wherein if the one or more sensedoperating conditions indicate that the power amplifier is operatingbeyond a specified temperature level, then the protection circuit causesthe power of the input signal to the power amplifier to be reduced. 9.The apparatus of claim 1 wherein regulating the power of the inputsignal to the power amplifier comprises varying the amplitude of theinput signal.
 10. The apparatus of claim 1 wherein the protectioncircuit regulates the power of the input signal passing through thepower amplifier if the combination of two or more of the sensedoperating conditions is beyond some desired operating level.
 11. Theapparatus of claim 1 further comprising: a variable gain amplifierincluding a control terminal, the variable gain amplifier coupled to theinput terminal of the power amplifier, the protection circuit coupled tothe control terminal to regulate the power of the input signal to thepower amplifier.
 12. The apparatus of claim 11 wherein regulating thepower of the input signal comprises reducing the amplitude of the inputsignal.
 13. The apparatus of claim 11 wherein the power amplifierincludes a plurality of amplifying stages.
 14. The apparatus of claim 13wherein the variable gain amplifier is coupled to the output terminal ofa first amplifier stage and to the input terminal of a second amplifierstage.
 15. The apparatus of claim 14 wherein regulating the power of theinput signal to the power amplifier comprises varying the power of theinput signal to the second amplifier stage.
 16. The apparatus of claim11 wherein the variable gain amplifier further includes: an inputterminal to receive an input signal; and an output terminal coupled tothe input terminal of the power amplifier.
 17. The apparatus of claim 11wherein the protection circuit further comprises: a differentialamplifier including a first input terminal, a second input terminal, andan output terminal, the output terminal coupled to the control terminalof the variable gain amplifier; a sensor device coupled between theoutput terminal of the power amplifier and the first input terminal ofthe differential amplifier to sense one or more operating conditions ofthe power amplifier and provide a corresponding voltage to the firstinput terminal of the differential amplifier; and a reference voltagesource coupled to the second input terminal of the differentialamplifier to provide a voltage corresponding to a desired operatingcondition for the power amplifier.
 18. The apparatus of claim 17 whereinif the voltage at the first input terminal of the differential amplifieris greater than or equal to the voltage at the second input terminal ofthe differential amplifier then the output terminal of the differentialamplifier provides a regulating feedback signal, otherwise no regulatingfeedback signal is provided.
 19. The apparatus of claim 17 wherein ifthe voltage at the first input terminal of the differential amplifier isless than the voltage at the second input terminal of the differentialamplifier then the output terminal of the differential amplifierprovides a first voltage level, otherwise the output terminal of thedifferential amplifier provides a second voltage level.
 20. Theapparatus of claim 17 wherein if the voltage at the first input terminalof the differential amplifier is less than the voltage at the secondinput terminal of the differential amplifier then the differentialamplifier provides zero volts at its output terminal, and otherwise theoutput terminal of the differential amplifier provides a voltageproportional to the voltage at the first input terminal.
 21. Theapparatus of claim 17 wherein the sensor device is a voltage sensingdevice to sense the voltage at the output terminal of the poweramplifier.
 22. The apparatus of claim 21 wherein the voltage sensingdevice is a resistor.
 23. The apparatus of claim 17 wherein the sensordevice is a current sensing device to sense the current passing throughthe power amplifier.
 24. The apparatus of claim 17 wherein the sensordevice is a temperature sensing device to sense the operatingtemperature of the power amplifier.
 25. The apparatus of claim 17wherein the reference voltage at the second input terminal of thedifferential amplifier is dynamically configured.
 26. The apparatus ofclaim 17 wherein the reference voltage at the second input terminal ofthe differential amplifier is preset to a desired level corresponding tothe maximum operating level at which the power amplifier is specified tooperate.
 27. A power amplifier comprising: a power amplifying means foramplifying an input signal; a sensing means for sensing one or moreoperating conditions of the power amplifying means; and a variable gainamplifying means coupled to the power amplifying means for regulatingthe power of the input signal passing through the power amplifying meansas a response to the one or more sensed operating conditions.
 28. Thepower amplifier of claim 27 wherein the variable gain amplifying meansregulates the amplitude of the input signal passing through the poweramplifying means if the one or more of the sensed operating conditionsare beyond a desired operating level.
 29. The power amplifier of claim28 wherein the variable gain amplifying means reduces the amplitude ofthe input signal so that the power amplifying means operates within itsspecified operating limits.
 30. The power amplifier of claim 27 whereinthe variable gain amplifying means regulates the power of the inputsignal before the input signal reaches the power amplifying means.
 31. Apower amplifier protection system comprising: a first sub-system tosense one or more operating conditions of a power amplifier; and asecond sub-system to regulate the power of an input signal to the poweramplifier as a response to the one or more sensed operating conditions.32. The power amplifier protection system of claim 31 wherein the secondsub-system reduces the amplitude of the input signal if a poweramplifier operates beyond a maximum specified operating limit.
 33. Thepower amplifier protection system of claim 31 wherein the secondsub-system offsets the DC bias point of the input signal if a poweramplifier operates beyond a maximum specified operating limit.
 34. Thepower amplifier protection system of claim 31 wherein the secondsub-system varies the input signal between two stages of a multi-stagepower amplifier.
 35. The power amplifier protection system of claim 31wherein one of the one or more operating conditions sensed is the outputpower of a power amplifier.
 36. The power amplifier protection system ofclaim 34 wherein the output power is sensed by a forward directionalcoupler.
 37. The power amplifier protection system of claim 31 whereinone of the one or more operating conditions sensed is the current of apower amplifier.
 38. The power amplifier protection system of claim 31wherein one of the one or more operating conditions sensed is the outputvoltage of a power amplifier.
 39. The power amplifier protection systemof claim 31 wherein one of the one or more operating conditions sensedis the operating temperature of a power amplifier.
 40. A method forprotecting power amplifiers comprising: sensing one or more operatingconditions of a power amplifier; comparing one or more of the sensedoperating conditions to specified operating limits for the poweramplifier; and varying the power of an input signal to the poweramplifier if one or more of the sensed operating conditions are beyondspecified operating limits for the power amplifier.
 41. The method ofclaim 40 wherein varying the input signal to the power amplifiercomprises reducing the amplitude of the input signal if one or more ofthe sensed operating conditions are greater than the maximum specifiedoperating limits for the power amplifier.
 42. The method of claim 40wherein one of the operating conditions sensed is the output voltage ofa stage of the power amplifier.
 43. The method of claim 40 wherein oneof the operating conditions sensed is the current passing through astage of the power amplifier.
 44. The method of claim 40 wherein one ofthe operating conditions sensed is the temperature of the poweramplifier.